The recording of data on a moving memory device, such a magnetic data storage media, is best accomplished by means of NRZ (non-return to zero) recording in which the data is self-clocking without separate clock signals. Thus, only data is recorded in the media and no space is "wasted" for the recording of clock signals. To accomplish such recording, the recording signal in a track comprises a sequence of regular intervals, wherein a recorded signal (such as a transition between magnetic polarities) appearing in an interval is designated as a "1" and the absence of a signal in an interval is designated as a "0". The location in each interval at which a recorded signal (transition) may appear or be absent from is called a synchronous location.
A method for encoding and detecting such data is a partial response maximum likelihood (PRML) data reproduction method. The data is coded into a run-length limited code and modulated in accordance with a partial response characteristic imposed on a time-related sequence of synchronous locations as in known in the art.
Various types of PRML recording are employed for various recording media. A type of PRML recording which is advantageously employed for magnetic recording media is Class IV PRML recording which employs a plurality of intermediate synchronous sample points for each transition location. The intermediate synchronous sample points are called synchronous write clock boundaries. Thus, the encoded data is recorded on a track as a sequence of variably spaced transitions having a particular relationship to the write clock boundaries.
The reproduction of the data as sensed at the readback transducer therefore requires that the readback signals be detected at the synchronous write clock boundaries. The maximum likelihood detection and decoding may then be conducted to recreate the original data. Ideally, the read channel waveform, when detected at the synchronous write clock boundaries, provides integer-type relationships to one another. Examples are -2, -1, 0, 1 and 2 for EPR4 (Extended Partial Response Class IV), and -1, 0 and 1 for PR4 (Partial Response Class IV).
PRML recording provides an excellent signal to noise characteristic and low error rate as compared to conventional peak detection of run length limited codes, therefore allowing the recording of data at higher recording densities. High density recording provides many advantages, such as higher capacity for the same types of data recording media.
Ideally, the track is moved past a readback transducer at a fixed speed, and the write clock boundaries are presented at a regular rate so that the recorded signals are appropriately aligned and spaced to allow generation by the readback transducer of equal and appropriately spaced electrical pulses for detection at the synchronous write clock boundaries by a sample clock for an ML detection channel.
Magnetic disk drives operate at fixed rotational speeds with the data recorded in a series of concentric tracks. Thus, the track velocity is relatively constant and the recorded signals are presented to the read channel at a regular rate, so that the sample clock provides samples at the write clock boundaries with only minor adjustments to the sample clock by a PLL (phase-locked-loop). This allows the ML detection channel to work optimally.
Conventional magnetic tape employs peak detection for decoding the data, rather than PRML. A difficulty of employing PRML with magnetic tape is that tape speeds are highly variable. Many magnetic tape drives access specific sections of the tape, possibly by stopping and reversing direction. For example, such magnetic tape drives may have a nominal velocity of 2 m/s and an acceleration rate of 2,000 m/s.sup.2. Also, the tape is simultaneously unwrapped from a supply reel, whose wrap radius is constantly being reduced, and wrapped onto a take up reel, whose wrap radius is constantly increasing, requiring that the reel motors constantly change speeds to produce approximately the same velocity across the readback transducer. Further, the tape does not always move across the readback transducer in a perfectly straight path, and may wander from side to side. Thus, the resultant velocity variations may be up to 15%.
Additionally, magnetic tape typically comprises a plurality of parallel tracks which are recorded and are read simultaneously. Thus, digital sample clocks may be employed and use global frequency averaging to control the clock timing, i.e., average the clocks across all the tracks. The digital clocks operate at a fixed frequency with a digital PLL to identify the transitions and maintain a phase lock on the input readback signal.
Typically, the read channel is sampled at a higher rate than the write clock rate. For example, the read sample clock operates at 1.25 times the write clock rate. In a digital channel, the read clock is generated by a fixed oscillator independently from the waveform that is being read from the tape. The PLL operates using this fixed oscillator to determine where the write clock boundaries occurred. The PLL does so without affecting the sample clock rate. Instead, it mathematically tracks the waveform that is being read from the tape.
Thus, the fixed samples are asynchronous and are independent from the synchronous write clock boundaries of the input signal, and present a highly variable relationship. No synchronous samples are therefore provided that could be used by an ML detector.